The invention relates generally to a method for operating a gas turbine during select operating conditions such as under-frequency operation through extraction of air from the compressor.
Large increases in the electrical power consumptive demand placed upon an electrical power distribution grid will tend to reduce the electrical operational frequency of the grid, causing an “under-frequency” event. For example, a heavy or sudden electrical demand may cause a particular power distribution grid having a nominal operational frequency of 50 Hz to momentarily operate at 49 Hz. In conventional electrical power generation systems that utilize one or more heavy-duty industrial gas turbine for supplying electrical power to the grid, the physical speed of each turbine supplying power to the grid is synchronized to the electrical frequency of the grid. Unfortunately, as the physical speed of a gas turbine decreases with other things being equal, its power output correspondingly decreases. Consequently, during an under-frequency event, a gas turbine will tend to output a lower power. In the past, a common practice in response to a power grid under-frequency event (occurrence) is to increase the firing temperature of the gas turbine to produce more power in an effort to maintain a predetermined level of output power. Unfortunately, such over-firing of the gas turbine may reduce the operational life expectancy of various hot gas path components within the turbine.
Grid code regulations typically require that power production equipment have the capability to maintain load during under-frequency excursions. Various regions around the world have different requirements that must be satisfied in order for power equipment to be considered compliant. Typically, gas turbine generators meet these requirements by increasing firing temperature to maintain generator output within requirements. Increases in firing temperature increase power output at a given pressure ratio, which works adequately when the gas turbine does not approach any operating limits such as maximum pressure ratio capability or maximum inlet guide vane (IGV) position. A firing temperature increase is typically achieved by an increase the fuel flow supplied to the combustor. All things otherwise equal, the increase in fuel flow results in a higher pressure at the turbine inlet, which in turn applies backpressure on the compressor. Eventually, adding more flow results in a compressor pressure limit, which typically is observed by limiting the flow through the turbine through the diversion of compressor discharge air to inlet (inlet bleed heating) and/or reduction of fuel flow (and consequently firing temperature). However, this method has limited capability to meet grid code requirements for cool ambient conditions and/or low Btu fuels (e.g. syngas) applications, due to operability limits encountered by the gas turbine compressor.
Some conventional gas turbines, used for power generation, incorporate variable inlet guide vanes (IGV). Such variable stator vanes provide the ability to adjust compressor airflow by changing incidence angle (i.e., the difference between the air angle and the mean line angle at the compressor blade leading edge) in the front stages of the compressor. These variable IGVs permit an acceptable compressor surge-free operation margin to be maintained. Typically, maintaining surge-free operation is a vital operational criterion of the compressor component for gas turbines.
Wickert et al. (U.S. Pat. No. 6,794,766) provides a method for over-firing of gas turbines equipped with variable stator vanes (blades) to compensate for power output during under-frequency events. Wickert utilizes the variable stator vanes to increase the amount of airflow consumed by the compressor component in a predefined manner so to preclude and/or minimize a decrease in the level of output power generated during a grid under-frequency event and maintaining a safe margin during such an event. However, not all gas turbines are equipped with variable stator vanes to permit employing such a technique. Further, this action alone may not be sufficient if the maximum vane position is reached and a pressure ratio limit is encountered simultaneously while attempting to increase output. In this situation, other action must be taken to alleviate the pressure limit.
It would therefore be desirable to utilize an operational method, which would improve the power output during select operations and result in improved grid code compliance during under-frequency operation.